Detection of nuclear bomb material is an urgent national priority. Nuclear weapons and their components can be transported easily in shipping containers, trucks, and rail cars. Rogue states and non-state adversaries could use clandestine delivery for terrorism or extortion, with little risk of detection. The US government has ordered that all cargo be scanned for nuclear materials at border crossings and shipping ports, but there is yet no suitable means for doing so.
Neutron radiation is a signature of plutonium, the key component of most nuclear weapons. However, neutron radiation can be shielded, greatly reducing the number of detectable particles. To detect a shielded weapon or a small portion of smuggled plutonium, the maximum information must be obtained from any detectable neutrons. In addition, the neutron detector must reject background radiation such as cosmic rays and gamma rays from various radioactive materials in the environment. Roughly 1% of the maritime containers entering US ports have detectable gamma radiation, primarily due to items containing bentonite clay, potassium, granite, and some lighting and electronic devices. Neutron emitters are far less common than gamma emitters in cargo. Only about 0.01% of the border shipments produce a detectable neutron emission, due primarily to radioactive sources for industrial inspections, well logging, and research.
Neutrons from plutonium typically have an energy of about 1 MeV, with a spread in energies from about 0.5 to about 5 MeV generally. Neutrons in that energy range interact with matter primarily by scattering from an atomic nucleus. For most nuclei, the scattering can be either elastic or inelastic depending on the nucleus and other factors. For hydrogen, however, only elastic scattering is possible since 1H has no excited nuclear states. In n-p scattering, a variable amount of energy, about half of the neutron energy on average, is transferred to the recoil proton. The proton emerges with an energy and direction that depend on the scattering angle. The recoil protons with the highest energy emerge in a direction closest to the initial neutron direction, as required by momentum conservation.
Gamma rays typically interact with matter by photoelectric absorption, Compton scattering, or pair production, each of which generates one or more energetic electrons (positrons being treated as electrons herein). Electrons with 1-2 MeV typically have a relatively low rate of energy deposition in matter, in contrast to the recoil protons which have a very high energy deposition rate. Accordingly, gamma-generated electrons have a much longer stopping range (stopping distance) than the neutron-recoil protons. Depending on the energy and the material, gamma-generated electrons typically travel many millimeters or even centimeters before stopping, whereas recoil protons typically stop in a few microns to a few tens of microns.
A directional neutron detector would be a valuable inspection tool by helping inspectors to localize a source of neutron radiation. Determining the neutron direction would greatly amplify the statistical power of each detection. For example, during a 60-second vehicle scan, two or three detected neutrons would probably not be sufficient to trigger an alarm, since background neutrons are always present from cosmic rays and environmental sources. But detecting two or three neutrons coming from the same place in the vehicle would certainly be suspicious, thereby prompting a secondary examination. For revealing neutron threats, the overall effectiveness of a directional neutron detector is about two orders of magnitude greater than a simple non-directional detector due to the localization of the source.
What is needed, then, is a neutron detector that indicates the neutron direction, focusing on the few-MeV energy range, suitable for scanning whole containers and vehicles at shipping ports and border crossings. Preferably such a detector would also enable improved scanning of personnel in a walk-through portal application, and would also lead to an improved direction-dependent neutron survey meter. The detector should have high detection efficiency for neutrons, yet have excellent rejection of gamma rays and other non-neutron backgrounds. Preferably the detector uses no scarce materials, and has low cost.